1. Field of the Invention
This invention relates to a digitally implemented control system for controlling a direct current (DC) motor powered from an alternating current (AC) power source, and more particularly to such a control system having a low current compensation controller for controlling the motor under discontinuous armature current conditions.
2. Background of Information
A common type of control system for a DC motor utilizes a thyristor power module or converter to gate selected portions of half cycles of power from an AC source to an armature of the motor. Typically, a second thyristor power converter controls field current of the motor. Gate signals for the converters are generated by an analog or a digital control circuit.
Under low load conditions, in response to an increased gate angle, the converter phases back gating of voltage pulses to the armature. With this phase back, a point is reached where the current applied to the motor becomes discontinuous. As the effective gain of the converter decreases when the current is discontinuous, gain compensation is applied to a current controller. A common method of detecting discontinuous current monitors average current in the armature circuit. Whenever the average current drops below a predetermined minimum value, it is presumed that a discontinuous current condition exists and current controller gain compensation is applied. However, the average current and predetermined minimum value are not an accurate determinant of discontinuous current.
A typical example of a digital control system is provided by U.S. Pat. No. 5,321,343 entitled "Digital Motor Control System". In the control system, plural parallel digital control loops each control a selected motor parameter (e.g., armature current, motor speed, motor torque, etc.) in response to an associated parameter signal. Each loop applies a selected control action, such as derivative and proportional control, to the associated parameter signal to generate an associated intermediate control signal. One of the intermediate control signals is selected for application to a common digital integrator which, upon integrating the selected intermediate control signal, produces a resultant control signal having the integral of the selected control action. The control system also detects discontinuous current conditions and monitors voltages across switches of a thyristor power converter. A discontinuous current mode signal is generated when a voltage is simultaneously detected across all of the switches of the converter. The mode signal is reset, to indicate a continuous current mode, when the voltages across any two consecutively fired switches connected to the same DC bus in the converter drop below a threshold value.
As is well known in the art, the step response of a current controller under discontinuous current conditions is very slow in comparison with the step response for higher load currents where the current is continuous. At continuous armature currents, a small change in gate angle produces a large change in armature current. In digital control systems, non-linear gain compensation in the current controller improves low current step response. However, an increase in current controller gain at low current provides only partial compensation and leaves room for improvement.
There is a need, therefore, for low current compensation motor control which accurately compensates for nonlinearity in a converter at low load currents.
There is a further need for a low current compensation controller which operates in parallel with a conventional thyristor armature current controller.